In devices that rely upon magnetic resonance, a direct current magnetic field is applied and a perpendicular alternating current magnetic field is applied for excitation. The alternating current creates a field that permits resonant spins to be detected in the presence of other spins. Resonance imaging is based upon the fact that images can be calculated from the detected resonance spins. In conventional magnetic resonance imaging (MRI), resonant spins are created in slices. With a plurality of slices, two and three dimensional images can be calculated.
Sensitivity and resolution limits are well-recognized in magnetic resonance imaging devices, and most efforts at improving imaging have been directed at improving detection signal to noise ratios. Example prior techniques to increase the signal to noise ratio include the use of ever higher magnetic fields, better amplifier technology, and application of more efficient pulse sequences and signal processing techniques, among others.
Inductive coils are most frequently used for detection in spectroscopic and imaging settings. Over the years, inductive coil technology has successfully kept pace with improvements in magnet designs. Also, inductive coils in magnetic resonance systems serve the dual important functions of providing both the AC magnetic field to excite the resonant spins in a sample and detecting the signal from a sample.
Other research efforts have sought to change the basic model of inductive detection used in magnetic resonance devices. Alternate detection techniques that have been researched include, for example, force detection, direct transfer of angular momentum, and energy from the spin population in magnetic resonance using micro-mechanical cantilevers. Additional research has been conducted on the flux-detection class of magnetic resonance sensing schemes such as superconducting quantum interference devices, Hall sensors, and superconducting resonators, as well as optical methods.
High resolution three-dimensional imaging is useful for microscopy, and can be used, for example, to study molecular structures. An eventual goal is making practical three-dimensional atomic resolution. Although improvements in imaging resolution through conventional inductive detection have steadily progressed within the last three decades, present spatial resolution is limited to approximately 1 μm in nuclear and electron spin magnetic resonance microscopy. The challenge in improving the imaging resolution results from the extremely weak signals in the magnetic resonance process, spin diffusion, and the limited ability to create sufficiently large gradient fields by current carrying coils.
A recently developed high resolution technique is Magnetic resonance force microscopy (MRFM). MRFM uses a microscopic magnetic particle as a source of atomic scale imaging gradient fields and a mechanical resonator as a sensitive detector of magnetic resonance. The progress in MRFM has recently culminated in the mechanical detection of a single electron spin. With MRFM, mechanical detection of a single electron spin magnetic resonance can be performed on a sample with spatially well-isolated spins, but only after significant averaging per data point, e.g., 13 hours or more. To reduce MRFM averaging time, research is presently focused upon the mechanical detector, magnetic field gradient source, and optical nanoreflectors used in the MRFM technique.
Sensitivity limits in magnetic resonance microscopy are being intensively pursued, primarily through changes to the type of detection that is used. In addition to more advanced micro/nano-mechanical force detectors, several other sensing mechanisms remain viable candidates for improving the imaging sensitivities in detection of small number of spins. These include, for example, the measurement using micro-mechanical cantilevers, flux-detectors such as micro-coils, superconducting quantum interference devices (SQUID), Hall sensors, superconducting resonators, and optical methods. Additionally, single or few spin detection schemes will likely require new methodologies in the area of quantum measurement that deviate significantly for the classical theory of magnetic resonance detection, and have to involve careful consideration of spin polarization and spin noise in a few-spins regime.
Single spin detection might soon be readily accomplished through signal-to-noise improvements under limited conditions. However, the signal-to-noise ratio improvements will be unlikely to succeed in common conditions such as normal imaging conditions. Typical imaging conditions are naturally dense spin environments. With conventional resonant imaging strategies, more than a single spin is resonant and neighboring spins contribute to the detected signal. Conventional resonant image strategies provide slice-selective resonance. Slice based resonance imaging is based upon the resonant condition:ω(r)=γ|B(r)|  (1)which defines a scalar relationship between the resonant frequency of the spin, ω, and the magnitude of the magnetic field, |B|, at the location of the spin, where γ is the gyromagnetic ratio for the nuclear or electron spin. It is the magnitude of the magnetic field at the spin location that determines its resonant frequency, and therefore the slices of constant |B| have to be well understood in order to deconvolve and reconstruct the image from the available data. Generally, due to the size of the polarizing field that must be applied to the sample, only the z-component of the magnetic field from the gradient sources is considered. However, that is an approximation only, and needs to be carefully reconsidered when lower magnetic fields are utilized.
Maxwell's equations place restrictions on the properties of magnetostatic fields in free space. It is impossible for the magnitudes of the components of the magnetic field vector BX, BY, or BZ to have a local minimum or maximum in free space. Additionally, the magnetic field magnitude, |B|, cannot have a local maximum, but it can have local minimum in free space. Localized minimums have been generated with current carrying structures and used in the fields of plasma confinement, neutral particle trapping, and levitation. Others have also proposed magnetic resonance imaging techniques that were based on different physical principles for creating what the papers termed as an imaging focus point, and relied on the magnetic field gradients produced by the three-dimensional current carrying wires. See, Damadian, et al., “Field Focusing Nuclear Magnetic Resonance (FONAR): Visualization of a Tumor in a Live Animal,” Science 194, 1430 (1976); Hinshaw, “Image Formation by Nuclear Magnetic Resonance: The Sensitive Point Method,” J. Appl. Phys. 47, 3709 (1976). The current carrying structures limit practical extensions of the technique.